Within Subjects Design and Threats to Internal Validity

Key Concepts of Within Subjects Design

General Characteristics of Within Subjects Design

  • Involves a single group of participants that experience all treatment conditions.
  • Sometimes referred to as a repeated measures design.
  • All participants partake in every level of the independent variable, ensuring that differences in scores are due to treatment conditions rather than individual differences.
  • Structure typically includes two conditions:
    • Control Condition
    • Experimental Condition
  • Various conditions can be experienced sequentially or mixed together within a single session.

Internal Validity Threats in Within Subjects Design

  1. Time-Related Factors

    • Influences from time of day, fatigue, or participant maturation that can skew results.
    • E.g., performance may vary based on whether a test occurs in the morning versus the afternoon.

  2. Environment-Related Variables

    • Changes in cognitive ability due to external factors during the study.
    • E.g., a sudden event affecting anxiety that wasn't controlled for may influence results.

  3. Common Time-Related Threats:

    • History: External events impacting one measurement but not the other.
    • Maturation: Systematic changes occurring within participants over time.
    • Instrumentation: Variance in measurement criteria over time, which may skew this data set.
    • Regression Towards the Mean: Extremes in scores gravitating towards the average on subsequent measurements.
    • Order Effects: Sequencing of treatments affecting outcomes (e.g., practice effects).

Counterbalancing Techniques

Importance of Counterbalancing
  • Balances participant exposure to treatment conditions to mitigate order effects.
  • Ensures all variations of conditions are fairly assessed to prevent skewed data.
Types of Counterbalancing

  1. Complete Counterbalancing

    • All possible orders of treatment conditions utilized.
    • Provides a comprehensive approach but is impractical with many conditions.

  2. Partial Counterbalancing

    • Utilizes limited sequences while ensuring all treatments are applied.
    • Desirable for balancing practical limits in study designs, often using a Latin Square structure.

Advantages and Disadvantages of Within Subjects Design

Advantages
  • Fewer participants needed compared to between subjects design (about one-third required).
  • Increased statistical power since individual variances are controlled by comparing participants to themselves.
  • Reduced influence from individual differences, enhancing validity.
Disadvantages
  • Potential for increased attrition rates as studies are typically longer, leading to participant dropout.
  • Vulnerability to order effects, possibly skewing results due to prior condition experiences.
  • Ethical concerns may arise for certain types of repeated treatments (e.g., drugs).

Match Subjects Design

  • Attempts to merge benefits of within and between subjects design.
  • Participants are matched on relevant variables while employing separate groups for different treatments.
  • Ensures balance in the characteristics of participants across different conditions.

Data Analysis Techniques for Within Subjects Design

  • Utilization of repeated measures t-test for two-condition designs.
  • Use of ANOVA for multiple treatment designs to evaluate functional relationships.
  • Examples include studies that yield results based on minor wording changes, tracking response variations across groups.

Conclusion

  • Successful applications of within subjects designs require careful consideration of potential confounding factors and appropriate methodological adjustments.
  • The value of counterbalancing, matching, and robust analytical techniques are crucial for achieving valid and reliable results in psychological research.